There is a great demand for infrared transmitting optical components such as windows, prisms, lenses, light pipes, and domes. Using infrared (IR) transmitting crystalline materials such as CaF.sub.2, NaCl, and KCl to fabricate these articles is not cost effective, since crystal cannot readily flow like vitreous materials to form various sizes and shapes, but must be grown using extremely slow processes.
Fluoride glasses which are transparent from 0.3 .mu.m in the UV to 7 .mu.m in the IR have recently emerged as leading candidate materials for IR bulk optic applications. These glasses generally contain large amounts of ZrF.sub.4, HfF.sub.4, ThF.sub.4, or AlF.sub.3 as the glass network former. The remaining components may include alkali-earth metal fluorides such as BaF.sub.2, CaF.sub.2, SrF.sub.2, etc.; alkali metal fluorides such as NaF, LiF, and the like; rare-earth fluorides such as GdF.sub.3, YbF.sub.3, and the like; or fluorides of Group III elements, such as LaF.sub.3, YF.sub.3, etc. For example, see U.S. Pat. Nos, 4,445,755; 4,346,176; 4,761,387; and 4,341,873.
Fluoride glasses are classified as very poor glass formers. Their high tendency to crystallize during melting and fabrication is generally accounted for by the features of their viscosity-temperature behavior. At the liquidus temperature T.sub.l, which ranges from 700.degree.-785.degree. C., the fluoride glass shear viscosity is as low as 0.1 poise. The low viscosity is retained upon cooling the melt toward the glass transition temperature, T.sub.g, at which point it increases rapidly. This wide temperature range between T.sub.l (wherein the melt is very fluid) and the temperature just above T.sub.g (where the melt is very viscous) does lead to rapid nucleation and growth in fluoride glass melts. This is why rapid quenching of the melt through the T.sub.l -T.sub.g temperature range is necessary to produce fluoride glass articles which are free of crystalline defects. The rapid quenching criteria becomes even more critical when the melt is exposed to inhomogeneous nucleation sites during the melting and fabrication process.
At present, there are three approaches being investigated for the fabrication of fluoride glass optical components. None of these is capable of producing high optical quality glass articles in large size on a regular or production-line basis. It should be noted that a fluoride glass window measuring 5 inches in diameter by 1 inch thickness is considered to be a large size window.
The first approach for fabricating fluoride glass optical components consists of melting, and then refining, a fluoride glass at about T.sub.l under a dry atmosphere of nitrogen or argon in a platinum or gold crucible, then casting or pouring the melt into a nickel or brass mold which is pre-heated to around T.sub.g to prevent thermal shock. Fast quenching of the melt can be achieved by heat dissipation through the metallic mold. The casting technique, however, gives rise to striae or density fluctuation in the finished glass product, especially for large size articles (cf. Cook et al., "Large Scale Melting of Fluorophosphate Optical Glass", in Proceedings for the 4th International Symposium on Halide Glasses, Monterey (Calif.), January, 1987). In addition, sharp edges or microscopic dents which often appear at the crucible lip will act as nucleation sites which accelerate the crystallization process in the glass melt. This phenomenon will results in crystalline defects inside the finished glass article.
The second approach for making fluoride glass optical components consists of plugging the bottom nozzle of the crucible during melting, and then unplugging and draining the melt into a metallic mold, which has been preheated to approximately T.sub.g. This technique seems to work well with stable glass such as the fluorophosphate glasses (cf. Cook et al.), but induces severe crystal formation when applied to fluoride glass. This is caused by various nucleation sites, as described earlier, which are confined around the crucible nozzle. To prevent nozzle induced crystallization, extremely fast quenching is required. The rapid quenching criterion is generally difficult to control and often results in cracking of the glass.
The third approach for preparing fluoride glass optical components consists of hot pressing a fluoride glass specimen into a desired shape using a closed die, as disclosed in U.S. Pat. No. 4,388,097. This process, however, requires large size and defect free fluoride glass blanks to start with. In addition, hot pressing of small pieces of glass blanks or powder together will result in trapped bubbles when the glass flows together.